This invention relates to autothermal reformers for converting air, steam and hydrocarbon fuel into a reformate, and more particularly to preheating the air and steam with the reformate exiting the reformer.
In fuel cell systems, it is known to use an autothermal reformer (ATR) to react air, steam and a hydrocarbon fuel (e.g. gasoline, methane etc.) to form a reformate containing H2, CO2, H2O, N2 and CO. The CO and H2O is subsequently converted to H2 and CO2 and the reformate is supplied to the anode side of an H2xe2x80x94O2 fuel cell. In the ATR, a mixture of the hydrocarbon fuel, steam and air pass sequentially through two reaction sections i.e. a first partial oxidation (POX) section, and a second steam reforming (SR) section. In the POX section, the fuel reacts exothermically with a substochiometric amount of air to produce carbon monoxide, hydrogen and lower hydrocarbons (e.g. methane). The hot POX reaction products, along with the steam introduced with the fuel and air, pass into the SR section where the lower hydrocarbons react with the steam to produce a reformate gas comprising principally CO2, CO, H2, H2O and N2. The SR reaction is endothermic and obtains its required heat from the heat generated by the exothermic POX reaction that is carried forward into the SR section by the POX section effluent.
It has been determined that the reformate exiting the SR section must have a temperature of at least about 650xc2x0 C. (preferably about 700xc2x0 C. to about 750xc2x0 C.) in order to suppress methane formation and thereby increase hydrogen production. One way of achieving such high reformate temperatures is to preheat the air and steam inputs to the ATR. Preheating not only adds needed heat to the system, but permits minimizing the amount of air that is needed which, in turn, increases system efficiency. The present invention provides improved system dynamic response by more closely coupling the heat needs of the ATR input flow (air and steam) with the heat in the ATR output flow (reformate), rather than relying on heat from a separate combustor located at the end of the system. Hence, in accordance with the present invention, such steam/air preheating is effected by heating the steam and air input streams to the ATR with heat extracted from the reformate exiting the SR section of the ATR. The steam is used as an intermediate heat carrier fluid such that reformate is used to heat the air (and steam) without the potential for the reformate and air mixing and reacting (i.e. should a leak develop within the heat exchanger) which could cause deterioration of the heat exchange incident to a localized heating of the heat exchange at the leak/reaction site. More specifically, the present invention contemplates a method of operating an autothermal reformer that converts steam, air and hydrocarbon fuel to produce a gaseous reformate having a first pressure and a temperature of at least about 650xc2x0 C. The method comprises preheating the steam and air with heat extracted from the reformate by first passing the steam and reformate into a first heat exchanger that transfers heat from the reformate to the steam which is maintained at a second pressure greater than a first (i.e. reformate) pressure so that if a leak occurs in the first heat exchanger the steam will flow toward the reformate. Thereafter, the steam is fed into a second heat exchanger that transfers heat from the steam to air flowing through the opposite side of the heat exchanger. The air in the second heat exchanger is maintained at a third pressure that is less than the second (i.e. steam) pressure, so that if the second heat exchanger leaks, the steam will flow toward the air-side of the heat exchanger. The net effect of this arrangement is that heat is transferred from the reformate to the input air via the input steam without having the reformate in direct heat exchange relationship with air across the wall of a heat exchanger. According to a preferred embodiment of the present invention, a third heat exchanger is provided between the ATR and the first heat exchanger to reheat the steam after it has given up some of its heat to the input air. In one embodiment of the invention, the first and third heat exchangers are contained in a common housing. In another embodiment, the first and second heat exchangers are contained within a common housing. In the most preferred embodiment, all three heat exchange processes are effected within a common housing to reduce start-up time and to minimize heat loss as well as heat exchanger volume, mass and cost.